Depending on the number of people sitting down for dinner, allow for roughly ½ lb of mussels (with shells) per person for an entrée-sized portion.
This is not a dish that does particularly well re-heated, in terms of the mussels. Buy only the quantity of mussels you intend to cook that evening, and ideally buy the mussels fresh on the day that you intend to cook the dish.
Do not take the risk of cooking mussels that are not alive. If the shells remain open after tapping on the counter or are broken, the mussels are dead and will likely make you and your diners sick.
That being said, don’t be afraid of cooking fresh, live, cleaned, closed mussels sold from purveyors you trust. As the mussels open their shells fully during the cooking process, the salty broth they contribute to the final sauce for the pasta makes the initial mussel cleaning process entirely worthwhile.
Avoid overcooking the mussels, when the shells of a mussel are fully opened, the mussel is done cooking and can be set aside and eventually tossed with the hot pasta when the remaining mussels are done cooking.